1. Field of the Invention
The invention relates generally to hydromechanical devices for metering the delivery and pressure of a fuel feed to a hydraulic servovalve unit which is driven by the fuel. The invention also relates to a fuel supply circuit comprising said device and to a process for controlling the pressure and delivery of the fuel to a servovalve unit.
2. Technological Background
In an aircraft engine the fuel is supplied to fuel injectors at which the fuel is burned to provide the propulsive power and the power for feeding engine auxiliaries. The fuel is also used as a cooling liquid in heat exchangers, and also to operate a plurality of hydraulic servovalves. For example, nozzles and other variable-geometry elements are operated by hydraulic rams which are driven by a supply of the fuel under pressure.
The servovalves are usually supplied from a central servovalve feed unit whose pressure must remain constant and whose delivery must be capable of being varied in accordance with servovalve requirements.
The supply circuit for the injectors, exchangers and central servovalve feed unit comprises a low pressure pump which raises fuel coming from the aircraft tanks at an initial pressure Pca to a pressure Pb, a high pressure pump which further raises the fuel to a pressure higher than that required for the injectors and the servovalve feed unit.
Unused fuel is recycled. The proportion of recycled fuel relative to consumed fuel must not be excessive since the fuel is heated by its passage through the high pressure pump and is therefore less suitable as a cooling liquid. The high pressure pump may be a pump whose delivery is determined solely by its speed of rotation. This speed is in turn a function of engine speed. The pump may also be a pump whose delivery is a function not only of its speed but also of another parameter which can be so controlled that a pump running at a given speed can provide different delivery rates.
As explained in U.S. Pat. No. 5,715,674 to Reuter, variable delivery pumps have the advantage over fixed pumps of being able to deliver a fuel flow rate theoretically adapted to the instantaneous delivery required for engine operation.
The term xe2x80x9ctheoreticallyxe2x80x9d is used because, as explained in the said Reuter patent, the pump response time is not negligible. More particularly, the servovalves introduce erratic delivery variations requiring a control bandwidth at frequencies incompatible with pump control capacities whether the pump is of the variable geometry kind or of the fixed delivery kind. Excessive response times may lead to decrease in deliveries to the engine fuel injectors and/or malfunction of the servomechanisms operated by the pressurised fuel.
To overcome this problem the Reuter patent provides a pump control valve having an outlet connected to an element for controlling the delivery of the pump. The operation of the valve is not explained and, in particular, it is not clear how the pump can provide a better transient response time as stated in the last two lines of column 6 and the first line of column 7 of Reuter. It is clear, however, that the valve is an essential element of a control loop aimed at reducing as much as possible the delivery of the pump. This delivery is theoretically maintained at a level slightly above the level necessary to cover the delivery demanded by the engine fuel injectors and the delivery demanded at any instant by the servovalves.
Although such a device is an improvement over the prior art it does not provide a response time short enough to meet the requirements of the servovalves controlling various elements of the engine.
The inventors have realised that the response time of variable delivery pumps is short enough for a control loop comprising such a pump to control the fuel delivery to the engine fuel injectors without major problems. The latter delivery varies in accordance with known laws in dependence upon engine speed and other parameters such as temperature and the external pressure, which it is possible to monitor continuously so that they are known at any instant. Since the engine is a high inertia device its speed of rotation varies slowly. Similar considerations apply to the other parameters affecting the delivery consumed by the injectors.
The invention is therefore based on isolating the injector delivery control loop from the servovalve feed control loop.
To this end, the delivery required from the high pressure pump at any instant needs to be the sum of the delivery required by the engine fuel injectors at that instant plus the maximum delivery which may be required by the servovalves.
The maximum servovalve delivery is the delivery which would be necessary if all the servovalves were simultaneously each to demand the maximum delivery consumable by that servovalve. The sum of the maximum deliveries to all the servovalves for a given engine speed is therefore a known constant, so that the delivery variations of the pump are solely the variations of the fuel injector consumption. Delivery to the fuel injectors is therefore readily controllable.
When the required portion of the pump delivery has been delivered to the fuel injectors, the remainder of the delivery is sufficient to meet all servovalve requirements in all circumstances.
The invention therefore provides a pressure and delivery control loop for fuel directed to a servovalve feed unit downstream of a fuel injector feed unit, comprising a fuel intake at a first pressure and at a constant controlled delivery and a control device for keeping the pressure of the feed unit constant whatever the consumption of said unit and the variations of the various fuel pressures in dependence upon the engine speed and upon the other parameters affecting the pressures.
In an embodiment of the invention the pressure control device comprises a chamber for providing this constant pressure. The chamber communicates with the feed unit and, by way of a variable cross-section outlet port, with a portion of the fuel circuit between the high pressure pump and the low pressure pump.
The cross-section of the outlet port is varied in dependence upon the position of a first shutter which moves around a variable equilibrium position. This position is a function of the pressure difference between a first pressure downstream of the low pressure pump and upstream of the high pressure pump and a second pressure upstream of the low pressure pump. The movements of the first shutter around the variable equilibrium position are a function of the fuel consumed by the servovalve feed unit.
The variation of the port cross-section around the variable equilibrium position is effected, as explained above, by means of a first shutter movable in response to two opposing forces. One of these forces is provided by the combined action of the second pressure and of resilient means, such as a spring, and the other force is provided by the action of the first pressure on the shutter.
Variation of the port cross-section around the equilibrium position in dependence upon feed unit consumption is effected by the movement of a second movable shutter which is movable in response to two opposing forces. One of these forces is provided by the pressure forces exerted by the fuel combined with resilient forces created, for example, by a spring. The other force is created by fuel pressure forces. One of these other forces is created by fuel at the pressure of the servovalve feed. Consequently, a decrease in the latter pressure reduces the latter force and therefore leads to a closing movement of the second shutter.
For the sake of simplicity and improved reliability, the two shutters of the chamber are merged to form a single element formed by part of a movable spool which variably masks the opening of the outlet port. The spool has two ends and an intermediate part forming a piston.
The spool piston divides a compartment into two half-chambers, namely a first half-chamber and a second half-chamber, the latter being at the controlled servovalve feed pressure. The first half-chamber communicates with the upstream side of a diaphragm, and the second half-chamber communicates with the downstream side of the diaphragm. The upstream side of the diaphragm receives fuel originating from the high pressure pump, preferably through a filter. A spring applies a pressure to the spool in a direction towards the second half-chamber.
The first end of the spool is subjected to the pressure Pca and its opposite end is subjected to the pressure Pb.
The forces acting on the spool and tending to reduce the opening of the outlet port are:
the force exerted by the pressure Pca acting on the first end of the spool;
the force of the spring; and
the force exerted by the pressure upstream of the diaphragm and acting on the
spool piston.
The opposing forces acting on the spool and tending to increase the opening of the outlet port are:
the force exerted by the pressure Pb acting on the second end of the spool; and
the force exerted by the pressure downstream of the diaphragm and acting on the spool piston.
The parameters on which a direct action is possible by design and which must be considered in order to obtain a constant controlled pressure in dependence upon servovalve delivery are:
the cross-sectional areas of the first and second ends of the spool;
the diaphragm cross-section;
the elastic coefficient of the spring; and
the area of the spool piston.
In summary, according to the invention, in a fuel supply circuit of an aircraft engine including fuel injectors and a plurality of servovalves, a low pressure pump for raising the pressure of fuel from a tank from a pressure Pca to a pressure Pb, and a high pressure pump and filter for further raising the pressure of the fuel to a pressure Psf at the outlet of said filter, there is provided a device for controlling the pressure and delivery of fuel to said plurality of servovalves comprising:
a compartment divided in a sealed manner to define a first end chamber, an intermediate chamber, and a second end chamber;
a spool movable in said compartment and having first and second end portions and an intermediate portion;
said first end portion being engaged in a sealed manner in said first end chamber;
said intermediate portion including a piston dividing said intermediate chamber in a fluid tight manner into first and second half-chambers;
means for communicating said first end chamber with fuel at the pressure Pca;
means for communicating said second end chamber with fuel at the pressure Pb;
a first outlet port in said intermediate chamber in communication with fuel at said pressure Pb;
resilient means disposed in said first half-chamber and acting to urge said spool in the direction of said second end chamber;
diaphragm means having an inlet and an outlet;
said inlet being connected to receive fuel at the pressure Psf and also being connected to said first half-chamber;
said outlet being connected to a fuel feed line to said servovalves and also being connected to said second half-chamber;
said spool being movable in said compartment in response to forces exerted on said spool by said resilient means and the fuel pressures applied to said first and second end chambers and to said first and second half-chambers to cause said piston to vary the opening of said first outlet port.
Preferably, the cross-sectional areas of the first and second ends of the spool are identical. Consequently, the force tending to increase the outlet port opening is proportional to the pressure difference Pb-Pca. This pressure difference is in turn proportional to the square of the engine speed. The delivery through the diaphragm is proportional to the pressure upstream thereof.
Consequently, the delivery passing through the control device and consisting of the sum of the delivery to the servovalves and the return delivery from the exit of the variable outlet port of the second half-chamber is proportional to engine speed, at least within a certain range. This characteristic is well adapted to the delivery necessary for the servovalves, whose requirements increase with engine speed.
The invention also provides a process for controlling the pressure and delivery of fuel to said plurality of servovalves of an aircraft engine, comprising the steps of:
controlling the high pressure pump delivery so that, whatever the engine speed, the delivery from said high pressure pump corresponds to the sum of the delivery required by said fuel injectors at said engine speed and the prevailing flight conditions and the delivery which would be required by said plurality of servovalves if all of said servovalves operate simultaneously using the maximum delivery usable at said engine speed;
placing a servovalve feed unit downstream of a fuel injector feed unit and in communication with a compartment having an opening which is variable in dependence upon the position of a movable closure member; and,
causing said closure member to move to vary said opening in response to opposing first and second groups of forces, said first group of forces acting in the closing direction and comprising a resilient force, a force proportional to the pressure Pca, and a force proportional to a first pressure, and said second group of forces acting in the opening direction and comprising a force proportional to the pressure Pb, and a force proportional to a second pressure less than the first pressure and at a substantially constant difference therefrom.